TY - GEN
T1 - Rapid growth of garnet within a metamorphic vein inferred from misorientation angle distribution of garnet porphyroblasts
AU - Okamoto, Atsushi
AU - Michibayashi, Katsuyoshi
PY - 2006/5/15
Y1 - 2006/5/15
N2 - The microstructure of garnet aggregates within a metamorphic vein are investigated to constrain the duration of vein formation. Garnet-bearing veins occur subparallel to the foliation of a host mafic schist in the Sanbagawa metamorphic belt, Japan. Microstructural observations using SEM, EPMA and EBSD reveal that numerous small garnets (10-100 μm diameter) coalesced to form large porphyroblasts within the vein. EBSD analysis of the porphyroblasts reveals that misorientation angles of neighbor-pair garnet grains within the vein have a random distribution. This contrasts with previous studies that found coalescence of garnets in mica schist leads to an increased frequency of low angle misorientation boundaries by misorientation-driven rotation. The random misorientation angle distribution indicates that (1) garnets within the vein grew so fast that they could not rotate within the vein, and thus (2) random orientations of garnets at the nucleation stage were preserved. On the basis of a simple kinetic model that assumes rotation of garnet is rate-limited by diffusion creep of matrix quartz, the time taken for a small garnet grain to rotate 1° is estimated to be 10 3 - 10 4 years. This duration may indicate the upper limit of the duration of garnet growth, and of vein formation.
AB - The microstructure of garnet aggregates within a metamorphic vein are investigated to constrain the duration of vein formation. Garnet-bearing veins occur subparallel to the foliation of a host mafic schist in the Sanbagawa metamorphic belt, Japan. Microstructural observations using SEM, EPMA and EBSD reveal that numerous small garnets (10-100 μm diameter) coalesced to form large porphyroblasts within the vein. EBSD analysis of the porphyroblasts reveals that misorientation angles of neighbor-pair garnet grains within the vein have a random distribution. This contrasts with previous studies that found coalescence of garnets in mica schist leads to an increased frequency of low angle misorientation boundaries by misorientation-driven rotation. The random misorientation angle distribution indicates that (1) garnets within the vein grew so fast that they could not rotate within the vein, and thus (2) random orientations of garnets at the nucleation stage were preserved. On the basis of a simple kinetic model that assumes rotation of garnet is rate-limited by diffusion creep of matrix quartz, the time taken for a small garnet grain to rotate 1° is estimated to be 10 3 - 10 4 years. This duration may indicate the upper limit of the duration of garnet growth, and of vein formation.
KW - Garnet aggregate
KW - Misorientation
KW - Sanbagawa metamorphic belt
KW - Vein
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U2 - 10.1063/1.2207097
DO - 10.1063/1.2207097
M3 - Conference contribution
AN - SCOPUS:33845577728
SN - 0735403279
SN - 9780735403277
T3 - AIP Conference Proceedings
SP - 167
EP - 170
BT - WATER DYNAMICS
T2 - WATER DYANMICS: 3rd International Workshop on Water Dynamics
Y2 - 16 November 2005 through 17 November 2005
ER -